The Navy SEAL team of military operatives who killed Osama bin Laden in a compound in Abbottabad, Pakistan on Sunday night was made up of some of the best-trained troops in the world. SEAL Team Six, the "Naval Special Warfare Development Group," was the main force involved in Sunday's firefight.

The daring operation began when two U.S. helicopters flew in low from Afghanistan and swept into the compound where Osama bin Laden was thought to be hiding late Sunday night Pakistan time, or Sunday afternoon Washington time. Thirty to 40 U.S. Navy SEALs disembarked from the helicopters as soon as they were in position and stormed the compound. The White House says they killed bin Laden and at least four others with him. The team was on the ground for only 40 minutes, most of that was time spent scrubbing the compound for information about al Qaeda and its plans.

The Navy SEAL team on this mission was supported by helicopter pilots from the 160th Special Ops Air Regiment, part of the Joint Special Operations Command. The CIA was the operational commander of the mission, but it was tasked to Special Forces.

U.S. Navy Sea, Air and Land Teams, commonly known as SEAL Teams, are the best of the best. Their creed is to be "a special breed of warrior ready to answer our nation's call."

"Everybody has got a dozen responsibilities and more importantly, and this is what separates these types of individuals with everbody else, they can do their job and if somebody else goes down they can fill right in and take over the additional job," Howard Wasdin, a former SEAL Team Six member who wrote a book about his experiences called "Seal Team Six" coming out May 24th, told Nightline's Terry Moran. "That just comes from years of training."

"We are reminded that we are fortunate to have Americans who dedicate their lives to protecting ours," President Obama said today. "We may not always know their names, we may not always know their stories, but they are there every day on the front lines of freedom and we are truly blessed."

In 2009, another SEAL team was instrumental in rescuing the American captain of the cargo ship Maersk Alabama from armed pirates off the coast of Somalia. On that mission, SEAL snipers fired perfect shots -- from the deck of a heaving ship -- to neutralize three pirates, with three bullets, simultaneously. The SEALs began their work in 1942, when military leaders decided to set up an elite team to scout beaches suitable for landing troops in World War II.

"There are other operations going on around the globe constantly," said Capt. Duncan Smith, a SEAL spokesman who spoke with ABC News.

These men have done all of this in anonymity. It is standard procedure never to identify members of Team Six.

"A lot of those missions -- a majority of those missions -- are ones that the public will never know about... and that's a good thing," Smith said.

Navy SEALs toil in the dark of night, tasked with the most daring, dangerous and important missions. To become a SEAL, those men completed some of the most brutal training regimens ever devised, designed to push the boundaries of even the most able service members. Only one third of recruits eventually become SEALs.

"You have to be able to endure a lot of physical pain and sometimes emotional pain, and you just have to dig deep. It's an elite organization and so it can't be for everybody," said Paul Tharp, master chief of the Naval Special Warfare Preparatory School and a SEAL for 24 years.

"What sets SEALs apart is our diversity in terms of the environments in which we operate," said Smith, also a SEAL for 24 years. "We operate at 10,000 feet in the Hindu Kush Mountains. We operate in desert regions in Iraq and elsewhere. We operate in jungles throughout the world."

As of 2009, there were 2,500 active duty SEALs. With the expanding war on terror and missions in 30 countries, the Navy needs more, but finding young men who can meet the SEALs' standards is a challenge.

"We are not looking for cocky kids," said Senior Chief Hans Garcia, a SEAL recruiter. "The perfect person would be a candidate who is remarkably physically fit, but is pretty humble, an analytical thinker, a problem solver -- someone who is very value-oriented, patriotic, puts service above self."